Projection devices for microlithography are used for producing microstructures on substrates, for example during the production of semiconductor elements. Thus, the structure of a reticle disposed in the object plane of a projection objective or of a mask disposed in the objective plane of the projection objective is imaged into an image plane, in which the substrate to be exposed (e.g. a wafer) is located.
The optical system of the projection objective typically includes several optical assemblies, each including a plurality of optical elements, like e.g. lenses or mirrors. An example of catadioptrical projection optics is described in the document U.S. 2003/0021040 A1. The disclosure of the document is incorporated herein by reference.
In immersion objectives for microlithography with apertures NA>1.0, the last image oriented optical element in front of the wafer is typically a planar convex lens, whose convex surface is configured almost semispherical. The convex surface of the planar convex lens is disposed object oriented (reticle oriented), the planar surface of the planar convex lens is disposed image oriented (wafer oriented). In other words, the planar plane of the planar convex lens forms the last transition surface of the optical system towards the image plane, and thus towards the substrate.
In order to avoid reflections, in particular total reflections, at the planar plane, an immersion liquid is provided between the last image oriented optical element and the substrate.
An embodiment of a typical configuration of an immersion objective is illustrated in FIG. 1, which depicts a terminal assembly 1 of an immersion objective, in which the immersion liquid 3 is disposed image oriented and adjacent to the planar plane 2a of a planar convex lens 2. The assembly includes an aperture 4.
The position of the optical element 2 in the optical system is such that the planar surface 2a is disposed proximal to the field, while the convex surface 2b is considered as proximal to the pupil. In this system, no optical element is provided most proximal to the wafer, for which both surfaces are not proximal to the pupil.
The FIGS. 2 and 3 emphasize that only the planar surface of the planar convex lens most proximal to the wafer can be considered as proximal to the field, while the convex surface of the lens 2 most proximal to the wafer has to be considered as proximal to the pupil. FIG. 2 shows the footprint of beam bundles 5 of various field points on the convex surface 2b of the lens 2. The beam bundles overlap strongly as it is to be expected for a surface proximal to the pupil. The intersection point 6 with the optical axis is disposed relatively central in the inner portion of the beam bundles 5.
FIG. 3 shows the footprint 5′ of the same beam bundles on the planar surface 2a of the lens 2, when the beam bundles exit from the lens 2. It is evident from FIG. 3, that the intersection point with the optical axis 6′ is disposed highly eccentric relative to the group of the beam bundles 5′. The fact that field points exist, whose beam bundles 5′ do not overlap, shows that the planar surface 2a of the optical element 2 is disposed proximal to the field.
A planar convex lens, which terminates the optical system, can be affected by the immersion liquid over time. In particular, contaminants, which are, for example, caused by the substrate surface, the photo lacquer or similar, are deposited on the lens, which degrades the imaging quality and the service life of the optical element.
In order to utilize the service life of the projection objective, replacing the terminal optical element is an option. However, it can be difficult and rather expensive to replace a planar convex lens, in particular when it is made of expensive materials. It has been proposed to introduce a planar plate as a terminal optical element close to the wafer, which can be exchanged easily, so that it can be replaced when the quality of the imaging deteriorates.
Immersion lithography objectives can undergo deterioration of the imaging quality through scatter radiation. Respective apertures have been used, for example between optical assemblies, to substantially prevent the impact of scatter radiation on the substrate.
Examples of known immersion lithography objectives and technologies can be found in WO 2006/128613 A1, JP 2004/246343 A1, US 2004/0001190 A1, JP 2005/086148 A, as well as in EP 1768171 A1.